Modern refrigeration systems provide cooling, ventilation, and humidity control for all or part of an enclosure. Such enclosures may include a refrigerator, a cooler, a vendor, a dispenser, and other types of light commercial or household appliances.
Because of environmental, financial, and other reasons, these modern refrigeration systems are increasing moving away from the use of synthetic refrigerants such as hydrofluorocarbons. Given such, there is an increased interest in the use of natural refrigerants such as carbon dioxide and the like. The use of carbon dioxide as the refrigerant may have the advantages of being relatively inexpensive, readily available, non-toxic, nonflammable, and environmentally friendly. Moreover, carbon dioxide generally has a higher volumetric capacity as compared to most common synthetic refrigerants.
Generally described, a carbon dioxide refrigeration cycle may be similar to other types of refrigeration cycles but may operate at higher pressures and may not involve a change in state. The typical supercritical carbon dioxide refrigeration cycle may include compressing the flow of carbon dioxide within a compressor at a high pressure and a high temperature. Second, the compressed carbon dioxide may be cooled within a gas cooler or other type of heat exchanger by heat exchange with the surrounding environment. Third, the carbon dioxide may pass through an expansion device that reduces both the pressure and the temperature. Fourth, the carbon dioxide may be pumped to an evaporator or a further heat exchanger where the carbon dioxide may absorb heat from an enclosure so as to provide cooling therein. The flow of carbon dioxide then may be returned to the compressor so as to repeat the cycle. Many variations on such a carbon dioxide refrigeration cycle may be known.
One way to improve the efficiency of a carbon dioxide refrigeration system is to use a two-stage compressor. The miscibility of oil in carbon dioxide in such carbon dioxide refrigeration systems, however, may be greater as compared to typical synthetic refrigerants at high operating pressures. Moreover, the miscibility of oil in carbon dioxide may increase as the pressure increases. Such an increase in the oil content of the refrigerant may present a challenge at the evaporator and elsewhere. Specifically, oil may begin to accumulate in the evaporator as the temperature of the refrigerant is reduced.
Moreover, the viscosity of the oil may increase so as to lead potentially to increased maintenance needs, premature failure of the components, clogging, and other types of ongoing maintenance issues.
Although different types of oil separators are known, such systems generally require pumps and/or complex valve arrangements due to the pressure differential between the inlet and outlet of the compressor. Moreover, such known oil separators may be ineffective with respect to two-stage compressors given that the oil is needed within a shell casing at an intermediate pressure.
There is thus a desire for an improved carbon dioxide refrigeration system for use with light commercial or household appliances and the like. Such an improved carbon dioxide refrigeration system may accommodate the increased miscibility of oil in the carbon dioxide refrigerant at higher pressures for an increase in overall system performance and efficiency with a reduction in maintenance requirements.